ENTROPIC FORCES BETWEEN AMPHIPHILIC SURFACES IN LIQUIDS

The forces between fluid amphiphilic surfaces such as surfactant micelles, lipid bilayers, and microemulsion droplets in liquids include the expected attractive van der Waals and repulsive electric double-layer forces (the two DLVO forces). However, because of the dynamic (fluid-like) nature of these interfaces, additional entropic fluctuation forces are also present which arise from the overlap of thermally excited surface modes. Three of these forces are the undulation, peristaltic, and protrusion forces. The first two arise from collective motions of bilayers or membranes and can be described in terms of their continuum elastic moduli, respectively. The last arises from molecular-scale fluctuations of hydrocarbon chains and other parts of the molecules protruding out of the surfaces ("molecular protrusion" force), and a similar osmotic repulsion between overlapping mobile headgroups ("headgroup overlap" force). It is shown that both these two forces are expected to decay roughly exponentially with distance with characteristic decay lengths in water of about 0.2 nm. These forces have long been believed to be due to water structure (and are commonly called the "hydration" force). By a review of recent experimental and theoretical progress in this area, it is concluded that this force is not primarily due to water structure (it occurs in other liquids than water) and that it is more akin to the steric repulsion between polymer-covered surfaces. Genuine hydration or solvation effects probably play only an indirect role in the interactions between amphiphilic surfaces, mainly in determining the hydrated sizes (excluded volumes) of the protruding groups and the positions of the planes of origin of other interaction potentials. A quantitative assessment is made of the relative contributions of DLVO forces, entropic forces, and genuine hydration forces between uncharged amphiphilic surfaces in water. It is concluded that between free bilayers the undulation repulsion dominates at large separations (> 3 nm), the van der Waals attraction at intermediate separations (1.5-3 nm), and the protrusion and overlap repulsions at smaller separations (< 1.5 nm). However, between bilayers with very long headgroups, the headgroup overlap repulsion may dominate at all separations. With this new interpretation many phenomena arising from bilayer interactions may now be better understood.